1. Field of Art
The present invention relates to apparatus for analyzing a sequence of video frames, a system utilizing such apparatus and a method of operating such apparatus and system. In particular, but not exclusively, embodiments of the present invention relate to video surveillance networks.
2. Description of Related Art
Video surveillance networks comprise one or more video cameras arranged to provide surveillance of a particular geographical location. The network may simply comprise a single video camera monitoring a portal which, for example, may be a door, a corridor, a lift, an entrance hall, an exit hall, a concourse, an airport security gate or anywhere there may be numerous individuals passing through. In some cases, such as an entrance hall for a large shopping center or a departures hall of an airport, there may be thousands of people passing through in a relatively short space of time. This makes tracking individuals who may be considered to be a threat or of interest to security in such cases fraught with difficulty.
Generally, video surveillance networks comprise a plurality of video cameras arranged to provide surveillance over a distributed geographical location such as an urban environment. Each video camera has a particular field of view or “view field” over which it captures and may record video images. A video camera may be static or may be remotely controllable to move its view field over the area under surveillance. A video image for a view field at a particular time is often termed a “scene” and may correspond to a frame of a video image captured by a camera.
Cameras in a network may be linked to a control center over a communications network using, for example, Internet protocol-based communications. Optionally, cameras may be linked to a control center via a dedicated analogue or digital communications channel. Control centers are usually manned by human operators, even when there are automated alert responses to particular scenarios within the network. The human operator needs to observe a camera which is showing a relevant incident, for example a scenario which has generated an alert. In networks comprising many cameras, the operator cannot monitor all the cameras, even when each camera has a dedicated display screen in the control center.
Additionally, many video surveillance networks cover a wide geographic area and may have non-overlapping or non-contiguous view field coverage. Additionally, cameras may cover crowded areas and may include objects which are occluded by other objects, for example static occlusions where an urban feature occludes a portion of a street thereby occluding pedestrians and vehicles passing behind the urban feature or where pedestrians are occluded by each other such as in crowded environments.
Monitoring such video surveillance networks may be highly complex, resulting in it being difficult to observe an incident and/or track activities leading up to an incident or following on from an incident.